The technological field of the present invention is refrigerated lamp-pumped solid-state lasers. Lasers of this kind usually employ rare-earths crystals, for example a holmium crystals. The following description, while referring in particular to only holmium lasers, introduces the needs and technological objectives to be achieved by the laser apparatuses of the present invention.
Laser holmium emits radiation at about 2.06 .mu.m, as a result of a transition between the .sup.5 I.sub.7 .fwdarw..sup.5 I.sub.8 electronic levels. Laser emission results from optical pumping to higher energy levels belonging to energy donors (or sensitizers) and a transfer of the excitation energy to the activator lasing ion.
A significant advantage of the Ho.sup.3 + for laser operation is the long lifetime of the emitting level .sup.5 I.sub.7, which results in high energy storage capability and efficient Q-switched operation (Chicklis, E. P. et al., Appl. Phys. Lett., 19:119 (1971)).
The first operation of the Ho:YAG laser with Er.sup.3+ and Tm.sup.3 + as sensitizers was reported by Johnson, et al. (ibid. 7:127 (1965); ibid. 8:200 (1966)). They obtained 7.6 W for 550 W of electrical input energy and calculated a total efficiency of about 5% by including geometric factors such as lamp and laser rod dimensions.
Beck and Gurs (J. Appl. Phys. 46: 5225 (1975)) reported output power of 50 W (slope efficiency of 5.6%) for Ho:YAG laser pumped in an elliptical cryogenically cooled cavity.
Early works in the field (Johnson, L. F. et al., J. Appl. Phys. 44: 5444 (1973); Mori, K. Phys. Stat. Sol. A42: 375 (1975); Devor, D. P. et al., J. Chem. Phys., 81: 4104 (1984) reported on power degradation (solarization) effects in YAG, while similar effects were not seen in holmiun doped YLF. The degradation effects were associated with the UV part of the pumping source, which led to color centers formation. This type of degradation was partly eliminated by the use of appropriate filters (Johnson, L. F., J. Appl. Phys: 44 5444 (1973)). Part of the work was aimed at elucidating solarization effects.
Two main problems are associated with the operation of holmium lasers:
1) Laser transitions in Ho.sup.3+ ion (as in other rare earth ions) are between f electronic levels. These are symmetry-forbidden transitions (Laport forbidden), having weak transition probabilities and oscillator strength of the order of 10.sup.-6. PA1 2) The Ho.sup.3+ laser at ambient temperature is a three-level system. The laser transition ends at the upper stark levels of the multiplet .sup.5 I.sub.8. This multiplet is populated at room temperature; therefore, a population inversion and a resulting decrease in the output power of the laser is expected.
One way to overcome problems associated with the weak transitions of rare earths ions is the use of energy donors (or sensitizers) such as Er.sup.3+ and Tm.sup.3+, utilizing their ability to absorb part of the pumping energy and transfer it to the Ho.sup.3+ ions.
The second problem requires a cryogenic cooling system for the laser rod. The cooling results in population of the lower stark splittings of the .sup.5 I.sub.8 level, leaving the higher multiplets (which are the terminal levels of the laser transition) unpopulated.
For elliptical cavity lasers, the choice of a particular cavity configuration usually depends on geometric factors (size of the laser rods and pumping lamp), physical requirements (maximum laser efficiency versus maximum laser uniformity) and on system considerations (cooling system, weight, compactness).
Another element which plays an important role in designing a cavity is the type of pumping source. Optical pumping is achieved by using a gas discharge lamp filled with xenon or krypton, or, alternatively, by a filament lamp, e.g. a standard tungsten-halogen filament lamp. The gas discharge lamp has sharp discrete emission lines, while the tungsten-halogen has a continuous spectrum typical of a black body radiating source. The continuous radiation originating from the tungsten-halogen pumping lamp yields significant portion which overlaps with the absorption spectra of Ho.sup.3+, Tm.sup.3+ and Er.sup.3+, which results in high pumping efficiency.
Most of the prior art laser cavities of the kind of a highly polished and chromium-gold plated elliptical cylinder, as used for any solid state laser, are made of two corresponding half cavities, which are assembled to form the cavity. Such an arrangement is satisfactory when the inner volume of the cavity may be flooded with water for cooling purposes, which is generally done in case of laser rods operating at or near ambient temperatures. In case of laser rods operating at or near cryogenic temperatures, this is not possible since the laser rod must be thermally insulated from the pumping lamp by a vacuum space.
Therefore, when operating at or near cryogenic temperatures the cavity and the rod must each be cooled separately. Since the cavity cannot be flooded, each half ellipse should be cooled separately, necessitating a plurality of piping connections for the cooling medium, generally water, involving a plurality of welds or soldered connections which are apt to become leaky with use. A drawback always present with elliptical cavities assembled from two halves is the difficulty in making matching complementary halves, as it is not possible to cut a one piece cavity into two halves, owing to the inevitable loss of material in cutting.
An additional problem with cavities consisting of two halves is the difficulty in achieving a sufficiently smooth polished surface, as the separate polishing of the two halves by conventional mechanical means presents complications, additionally there are problems of sealing the two halves of the elliptical cavity and special arrangements are always required.
The laser rod holder has to be especially adapted for operation at very low temperatures (for example, under liquid nitrogen cooling), wherein a tight joint must be maintained between the circulating coolant and the surrounding vacuum and, additionally, provisions must be made to accomodate differential thermal expansion between the crystalline laser rod and the other construction materials. An additional requirement is to provide for flow conditions preventing bubble formation in the coolant flow, while maintaining turbulant flow conditions, thus providing for efficient cooling. In order to avoid stagnation in flow at some regions which may cause elevated temperatures in said regions it is mandatory to provide for smooth gradual transitions between different cross sections, therefore disassemblable connections comprising O-rings of different materials, including indium, are unsatisfactory. At the same time, there must be provisions for replacing the laser rod, without damaging the same. As will be shown hereafter, the laser rod holder of the claimed construction takes account of all the above requirements. These requirements have not been solved by prior art arrangements. The use of a thin walled copper tube for holding the laser rod and for relieving stresses originating from differential thermal expansion, and also the sealing of the rod to this copper tube is known from Liquid Nitrogen Cooled Laser Rod Holder Design, Gehemy, D. et al., Rev. Sci. Instrum. 51 (9), September 1980. However, the construction described in this publication involves disassemblable connections comprising O-rings, in particular of indium, and step transitions in the cooling liquid nitrogen flow, giving rise to the possibility of bubble formation, detrimental to an efficient cooling. Also the replacement of the laser rod is complicated.